The cutting of yarn strands bonded to a fabric sheet is required in the manufacture of various pile fabrics. Among these may be mentioned in particular, velvet, a combination known commercially as VELCRO and comprising a hooked fabric for fastening to a looped fabric, and certain machine-manufactured carpets. In each case, the cutting of these strands results in considerable knife wear.
It has recently been proposed to use a laser beam for cutting determined portions of yarn strands to form hooks on a band of looped fabric of approximately 5 cm in width. Two variations of the proposed solution are envisaged. One of these variations consists of transmitting a laser beam transversely to the band of fabric, focused in the contral zone of the band at any given instant to cut one of the branches of the loop. The other of these variations consists of directing the laser beam in the direction of the band and interposing a perforated mask to allow only part of the beam to pass. The results obtained by these processes are very poor because a large part of the energy of the laser beam is lost, so that in the case of plastic (synthetic resin) material the yarn is cut by fusion rather than by combustion. Consequently, the cut is not clean and the efficiency is very low. Moreover, such processes are suitable only for a very narrow band and are consequently unusable for wider fabrics such as those produced for garment manufacture, such as velvet.
The cutting of a piece of fabric to obtain velvet constitutes one of the most delicate operations in the manufacture of this material. In the case of the finest velvets, this operation is carried out with the aid of a knife fixed to a guide engaged in a column of fabric races or "floats."
These columns consist of yarn strands disposed as transverse loops aligned to form columns or ribs which are disposed side by side on one surface of the fabric web. An endless band is formed by sewing the two ends of one piece of fabric in such a manner that the end of each column coincides with the end of an adjacent column, the knife is introduced at one end of a continuous column so formed, and the fabric band is made to move along so that all the transverse strands are cut. This is repeated until all the columns of races have been cut. This operation involves about twenty hours of work for a piece of a size 300 meters.times.0.070 meters. The normal wear of the knife sometimes causes the loss of the piece of fabric, or at best, its sale as a remnant. In this respect, a knife change during the cutting of any one piece of fabric leads to an apparent modification of the velvet reflection. The resultant loss of value is considerable.
This method of cutting velvet also suffers from not being able to be used in the cutting of synthetic yarn, so that nearly all the velvet at present produced in this manner is cotton, the knives used being unsuitable for cutting a piece of synthetic fabric. In addition to these disadvantages, the use of a knife constitutes an obstacle to increase of cutting speed, which is limited to between 3 and 5 meters/second.
The above-identified applications describe an apparatus capable of severing thread loops disposed in parallel columns side by side on one of the faces of a fabric web in which the severing operation is effected by means of a laser beam. Essentially, this apparatus comprises guide means constituted by a needle adapted to pass through the thread loops of a column and rigid with a support, means for guiding the support substantially transversely to these columns, a lens for focusing the laser beam fixed on the support and having a focal point located along the guide needle and drive means for relatively displacing the laser beam and the loops forming the aforementioned column.
In this system, while considerable energy of the laser beam is conserved, it has been found that there is nevertheless a loss of energy. Generally, speaking, the thread loops, for the formation of velvet, i.e. the so-called floats of velvet, are formed by the weft threads. Consequently, the thread loops to be cut are spaced from one another by a distance which is approximately equal to the thickness of the thread to be cut.
When the columns of thread loops are displaced along the needle to the laser beam focal point, therefore, the individual loops are spaced apart and aproximately half the laser beam's energy is lost. This loss of laser beam energy corresponds to the time during which the laser beam is trained upon a bare spot of the needle, i.e. the time between passages of the thread loops into the focal point. As a result, the needle is subjected at a fixed point to considerable laser beam energy and tends to become weakened and to break.